I think that since this moment, Sangwoo can’t trust the words “I love you” (or even I like you) anymore. He thinks they are empty words that can mean nothing or be fake and he doesn’t trust anyone who tells him those words because he associates them to infidelity, lies and betrayal.
That’s why he is constantly testing Bum and felt so betrayed when he tried to escape in season 1 after he told him that he loved him.
And also when he hid Seungbae.
That could have proved to him again that those words are actually meaningless, empty and fake. That’s why he told Bum to shut up when he told him he liked him in chapter 35.
He’s afraid of Bum betraying him, lying to him or even cheating on him after having told him so many times that he loves/likes him (maybe that’s why he started being more openly jealous and possesive with him)
But in this season, he probably started to realize that Bum really loves him and won’t leave him despite everything he has done to him.
He thinks he has found someone that will stay with him no matter what, someone whose “I love you”s aren’t fake and that’s why he can’t let him go.
I guess it’s called character development.
I must say I still don’t trust Bum, I still think he can betray Sangwoo at any moment but I’m talking from Sangwoo’s POV here.